Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
~ 6S2~ CIL 595
This invention relates to blends of eithfsr low
density polyethylene or ethylene vinyl acetate (EVA) copolymers
with certain polymers of butene and to films made from such
blends that are suitable for usfa as self-sealing stretch
wrapping film and se~lf-sealing packaging film.
One technique for unitizing packages such as pallet
loads is known as rotary stretch wrapping. This technique
uses film which is wound under tension around the package by
either rotating the package on a turntable or by winding the
film around a stationary packagef, The films used for this
technique are made from a variety of polymers including low
density ethylene homopolymer (LDPE), athylene vinyl acetate
~ (EVA) copolymers containing from 2% to 12% hy weight vinyl
f~ acetate and plastized polyvinyl chloride (PVC). The preferred
characteristic of film used in rotary stretch wrapping is the
ability to cling to itself sufficiently thereby self-sealing
the wrap without the use of an adhesive or a heat sealing
f' ' operation. Other important characteristics of film used in
rotary stretch wrapping are toughness, stretchability and stress
retention,
,;~ Self-sealing packaging films are used for wrapping
display foods such as fresh meat cut~, bakery products and
~i~ produce in supermarkets and for wrapping of various foods in
~; the home such as sandwiches and salads, Self-sealing packaging
films are m~de from a variety of resins including ~DPE, EVA
~-~ copol~mers containing from 2% to 12% by weight vinyl acstate
and plastized PVC. The most important characteristic of self-
~ sealing packaging films is its ability to cling to itself suf-
f' ficiently to provide a wrap which does not require the use of
~,~ 30 an adhesive or a heat sealing operation.
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A proper degree of cling or tackiness in a film is
obtained by careful selection of the basic resin and of modi-
fiers therefore. In Japanese pre-puhlished application ~o.
49-54472 published on 27th May, 1974, there is proposed a
film suitable for stretch wrapping of food products, made from
low-density polyethylene or EVA copolymer (containing not less
than 70O/o ethylene) modified with a mixture of a polybutene
having a molecular weight of 1100-1500 and another polybutene
having a molecular weight of 100~500 This blend, although
affording cling-stret~h films of acceptable properties, suffers
from the disadvantage that it requires the handling and com-
pounding of two modifiers, namely two different grades of
polybutene.
It is an object of this invention to provide blended
polymeric ole~inic materials of superior properties Another
object is to provide such blended materials which may be easily
compounded in that they are comprised of a basic polymeric
olefinic resin and a single modifier.
The blends of this invention comprise a) from 90.0
to 98.0 per cent by weight of a polymer selected from poly-
ethylene and copolymers of ethylene with from 1~0 to 9.0
per cent by weight of vinyl acetate, having a melt index (MI)
in the range of 0,3 to 7.0 grams per 10 minutes and a density
....
in the range of 917 to 933 kg/m3, and b) from 10.0 to 2.0
per cent by weight of polybutene having a moLecular weight of
700 to 1000.
~- Any conventional method affording products of
~- suitable density, melt index and/ox molecular weight may be
~; used to prepare the two components of the blends. Poly-
ethylene having a melt index of about 2 g/10 minutes and
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a density of about 924 kg/m and EVA copolymer containingabout 2,0 per cent by weight VA have bean found to provide
the best results,
The blends of this invention may be prepared by
many known blending methods. The two polymers may, for
example, be suitably blended by mixing a-t a temperature above
their softening points on a device such as a roll mill or
a Banbury mixer or an extruder mixer, Additives such as anti-
oxidants and antifogging agents may be incorporated at the
compounding stage.
Transformation of the blends into films with the
desired cling and tac~iness can be accomplished either by
conventional blown tubular film extrusion or by chill roll
cast film extrusion. Film extruded by the blown tubular
method can be cooled by air blowers where the air is applied
either externally to the bubble, circulated within the bubble
or a combination of both, The film can also be cooled by
passing the extruded film (tubular ox flat) through a water
bath system, The water bath system or the chill roll cast
method produces films that are more tacky, more elastic and
have better optical properties.
Film extruded by the blown tubular method is normally
slit along both edges thereby producing two sheets o film.
The two sheets are then separated and wound individually.
The disadvantage of previously known films which exhibited
cli~g and tackiness is the difficulty of separating and winding
the two sheets of film. The films made from the blends of
this invention exhibit a delayed development of tackiness
such that the ultlmate tackiness is not achieved until ap-
proximately 24 hours after extrusion. This delay in develop-
ment of tackiness allows for easier separation of the two
sheets of film and thus easier handling during extrusion.
Films of suitable thickness are thus obtained which~
by virtue of their cling property are particularLy useul
in rotary stretch wrapping
During rotary stretch wrapping, 2 or more layers
of films are overlapped. The cling characteristic o the
film made from the blends of this in~ention heIp it to hold
the film layers together by adhering one layer of the wrap
to the next This inter-layer adhesion allows the multi
layers of wrap to behave as a sin~le, thicker layer thereby
reinforcing it and in~rea9ing its toughnessO
EXAMPLE 1
A series of nine blends were prepared using varying
amounts o polyethylene or EVA copolymer and poly~utenes with
the view to illustrating the above-defined ranges of melt
index, density, molecular weight and VA cont~nt. Each blend
was thereater extruded into tubular blown film about l mil
thick~ The cling property of each ilm was then dete~mined
~0 by the method described in A.S T.M. D-3354. The particular
features of each of the nine blends and the cling property
of the films obtained are ~hown in the following ~able
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T A B L E
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Blend Poly~utene Cling
~o Ba~ic Polymer ~ . % by wei~ht Property
. . ~ . . ~ewtons)
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1 EVA copolymer 5% 0,16
3.5% VA MW 900
MI 0.8, d. 925
2 Polyethylene 2% 0.13
. MI 2.0 MW 900
d. 918
3 Polyeth~lene 5% 0.70
MI 2.0 MW 900 . .
~. 918 ::
4 EV~ copolymer 5% lo 60
2% ~A, MW 900
MI 2.0, d. 920 ~ :
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EVA copolymer 4% 1.50
6,5% VA, MW 900
MI 3.0, d. 931
6 . EVA copolymer 10% 2.0 ~
2% VA MW 900 .,
MI 0.3, d. 925
: : 7 EV~ copolymer ~ 10% 2.0 ~
: 3.~5% VA MW 750 ~: ~ :
MI ~.0,: d. 926 :
. : 8 EVA copolymer ~ . 2,3
9,0% VA ~MW 900
MI 3,0, d. 931 :
: : 9 ' Polyethylene 5% 1.6
d. 9i7 MW 900 :;
_ _ __ _ . . __
MI = melt index
: d = densLty
: ~ MW = molecular weight
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SUPPLEMENTARY DISCLOSURE
In the principal disclosure there is described
a novel polymer blend comprised of speci~ically defined
components (a) and (b) the latter consisting of a polybutene
having a molecular weigh-t of 700 to 1000, It has now been
found that butene polymers having a molecular weight of
1000 to 2000, ~specially 1300 are also suitable for use
in the blends of the invention,
There~ore the scope of the invention is extended
to polymer bLends comprising
a) from 20,0 to 98,0 per cent by ~eight of
a polymer selected from polyethylene and
copolymers of ethylene with from 1,0 to 9.0
per cent by weight o~ vinyl acetate, having
a melt index ln the ran~e of 0~3 to 7.0 grams
: p~r 10 minutes and a density in the range of
917 to 933 kg/m , and
b) from 10,0 to 2,0 per cent by weight of poly-
butene having a molecular weight ~f 700 to
2000.
The following example is provided to illustrate
the above.
EXAMPLE 2
A blend was prepared with the following composition:
Polyethylene 96% by weight
Polybutene 4% by weight '`
The polyethylene was a conventional film-forming polymer
with MFI 2.3 and density 921 kg/m3. The polybutene had
a molecular weight of 1300 and it was derived from isobutene,
i,e,
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065'~:~
CH2 = C~CH3)2
These items were mixed using a conventional internal bladed
mixer.
` The blend defined above was conventionally con- :
;: verted into tubular film and slit into slit flat film 25
microns thick, This film was used on a commercial rotary
stretch wrap machine~ It performed satisfactorily adhering
to itself without the ~se o~ ~ny adhesive,
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